Sensitivity of an urban sub-grid scheme to the canyons orientations over a wide metropolitan area

In order to improve and incorporate the urban heat island (UHI) effects within mesoscale atmospheric models, Masson, V. (2000) proposed the physically-based model TEB (Town Energy Balance). In accordance with Porson, A. et al. (2009), the TEB can be defined as a three-facet model (hereinafter TFM), in that it utilizes three surfaces (roof, wall and road) to describe the canyon geometry. Unlike the TFM, the four facet model (hereinafter FFM), in the radiative and thermal budget, takes into account the facing walls of the canyon and their mutual orientation as well. Porson, A. et al. (2009) showed that the TFM exhibits a good agreement with the FFM, except for an error that comes from considering the average orientation of the urban canyons instead of the actual ones. However, the amplitude of the error associated with the TFM description on a wide urban area, (i.e., the domain of influence of a surface node in the computational grid) where the canyons show random orientations is still not well known. In order to investigate how much the averaging over the street canyon orientation affects the accuracy of the energy budget, an urban sub-grid scheme based on TEB was coupled with the Regional Atmospheric Modelling System (RAMS, Pielke, R.A. et al. 1992). Several model runs were then performed to simulate typical summertime atmospheric conditions as most favourable to observe the UHI effects over the urban area of Rome (Italy). The use of TEB-FFM, in association with a subgrid scheme, which takes into account each single canyon in the domain of influence of a grid node, effects significantly the sensible heat flux at the surface. The analysis allows us to ascertain that, for the purpose of atmospheric modelling, the choice of a proper urban sub-grid scheme must be supported by the knowledge of the detailed urban surface texture.